1. Field of the Invention
The present invention relates to an optical transmitter and an optical wireless transmission system for transmitting data to a terminal by means of optical space transmission using an infrared beam, and more particularly to an optical transmitter and an optical wireless transmission system for performing high-rate data communication by adjusting an optical axis by moving visually a terminal into a data transmission possible range of the optical transmitter.
2. Description of the Background Art
An optical wireless transmission system which performs data communication by emitting an optical beam to a free space enables wireless connection between devices such as an information processing terminal and audio visual equipment, and the like in an office and a house. Since there is no legal restriction on usage of frequency in comparison with a wireless communication system such as a wireless LAN device, UWB, and the like, the optical wireless transmission has advantages such as that high-rate communication can be achieved using wide band characteristics of light. Thus, the optical wireless transmission system has been attracting attention in recent years. Recently, with a further increase in capacity of data communication, there is a need for an optical wireless transmission system which enables higher-rate communication.
For performing high-rate data communication between an optical space transmission apparatus which transmits data and a terminal which receives data (between the devices) by means of optical space transmission, a light receiver of the terminal generally needs higher light reception power in comparison with light reception power for performing low-rate data communication therebetween. A method is used in which the light reception power of the light receiver is increased by narrowing the directional characteristics of an optical beam which is transmitted from an optical transmitter. In this case, it is necessary to perform optical axis adjustment between the optical space transmission apparatus and the terminal. However, since a semiconductor laser of excellent high-rate modulation characteristics which emits an infrared beam, or the like is generally used in the optical wireless transmission system which enables high-rate communication, a user cannot look at a laser beam (the infrared beam). Thus, when the user performs communication while holding a terminal with his or her hand or the like, it is hard for the user to manually perform the optical axis adjustment of the terminal.
There is, for example, a method of performing optical axis adjustment in which an optical space transmission apparatus automatically searches a terminal by an optical transmitter of the optical space transmission apparatus emitting a beam in a wide range. There is, for example, a method of performing optical adjustment in which a lens is moved mechanically by MEMS (Micro Electro Mechanical Systems) or the like so that a maximum amount of received light is obtained by the light receiver of a terminal. Also, as a simple method of performing optical axis adjustment, there is a method using a visible light as follows (e.g. refer to Japanese Laid-Open Patent Publication No. 2005-101853).
FIG. 13 is a view showing a configuration of a conventional optical wireless transmission system 100 disclosed in Japanese Laid-Open Patent Publication No. 2005-101853. As shown in FIG. 13, the optical wireless transmission system 100 includes a master unit 101 and a slave unit 102. The master unit 101 includes a transmission section 103 and a reception section 104 on a base 111 thereof, and the slave unit 102 includes a transmission section 105 and a reception section 106 on a base 112 thereof. The transmission section 103 of the master unit 101 includes an infrared light emitting semiconductor laser (not shown) which emits a transmission light 108 as an infrared communication light, and a visible light emitting diode (not shown) which emits a visible light 107 which has the substantially same emission angle as that of the transmission light 108 and is incoherent with respect to the transmission light 108. The reception section 106 of the slave unit 102 includes a condenser lens (not shown) which converges the transmission light 108, and a light receiving element such as a photodiode or the like which receives the transmission light 108 converged by the condenser lens and converts an optical signal into an electric signal
The optical wireless transmission system 100 emits the visible light 107 as well as the transmission light 108 from the master unit 101 toward the slave unit 102, and a user visually inserts the reception section 106 of the slave unit 102 into a range in which the visible light 107 is emitted. Since the emission angle of the visible light 107 is substantially the same as that of the transmission light 108, the slave unit 102 can reliably receive the transmission light.
As described above, according to the conventional optical wireless transmission system 100, the user can recognize a transmission range of the transmission light 108 by looking at the emission range of the visible light 107. Thus, even though the transmission light 108 is emitted in a relatively small range, the optical axis adjustment can be performed, and communication is enabled, with the result that high-rate optical communication is possible with low power consumption.
In the optical wireless transmission system 100, however, a reflection portion need to be provided in the slave unit 102 for the user to look at the visible light 107 emitted from the master unit 101. When holding the slave unit 102 with his or her hand for performing communication, the user needs to look into a reflection portion which is mounted in a direction in which the master unit 101 is located for looking at the visible light 107 emitted to the slave unit 102.
As a solution to the problem, there is a method in which a visible light is emitted from a terminal (e.g. refer to Japanese Laid-Open Patent Publication No. 2005-339025). FIG. 14 is a view for explaining a configuration of a conventional content delivery system 200 disclosed in Japanese Laid-Open Patent Publication No. 2005-339025, FIG. 14(a) shows a configuration of the content delivery system 200, and FIG. 14(b) shows a configuration of a terminal transmitter 203 included in the content delivery system 200. It is noted that although the content delivery system 200 performs high-rate data communication using a extremely-high frequency, since the extremely-high frequency has high directivity (nature of straight travel) similarly to light, in the content delivery system 200, adjustment of an axis of the extremely-high frequency is considered similar to the optical axis adjustment in the optical wireless transmission system.
As shown in FIG. 14(a), the content delivery system 200 includes a terminal transmitter 203 and a portable receiving terminal 202 which includes a reception section 225 and an index laser 229. As shown in FIG. 14(b), the terminal transmitter 203 includes a light guide 215, a transmission section 211, an index sensor 222, and a light 223 for visual confirmation. The light guide 215 restricts a position where the user can look at the light 223 within a reception possible range of the extremely-high frequency transmitted from the terminal transmitter 203.
The user of the content delivery system 200 moves the portable receiving terminal 202 to a position where the user can look at the light 223 without disturbing the light 223 of the terminal transmitter 203 by the light guide 215, and presses an operation button which causes the index laser 229 of the portable receiving terminal 202 to emit a beam, and adjusts the facing direction of the portable receiving terminal 202 to irradiate the terminal transmitter 203 with the beam of the index laser 229. When the index sensor 222 of the terminal transmitter 203 receives the beam of the index laser 229, the transmission section 211 of the terminal transmitter 203 transmits contents (data) to the portable receiving terminal 202 using the extremely-high frequency.
By the configuration and the operation as described thus, the conventional content delivery system 200 can solve the described problems of the conventional optical wireless transmission system 100 (see FIG. 13).
In the conventional content delivery system 200, since the light 223, the index sensor 222, and the transmission section 211 cannot be located at the same position, the positions of the light 223, the index sensor 222, and the transmission section 211 are different from each other (see FIG. 14(b)). Thus, a range in which the index sensor 222 receives the beam of the index laser 229 is appropriately restricted by the light guide 215 to a range (hereinafter, referred to as transmission possible range) in which data can be transmitted from the terminal transmitter 203 to the portable receiving terminal 202. FIG. 15 is a view for explaining a relation between the transmission possible range and a range in which the index sensor 222 can receive the beam of the index laser 229 in the conventional content delivery system 200. As shown in FIG. 15(a), the difference between a transmission possible range 230 and a range 240 in which the index sensor 222 can receive the beam of the index laser 229 occurs so as to generate a region 250 indicated by diagonal lines which is included in the range 240 but not included in the transmission possible range 230. In the case where a beam of a narrow directivity angle is used for data transmission in order to perform data transmission at a higher speed, as shown in FIG. 15(b), the region 250 indicated by the diagonal lines is relatively large in a relation with the transmission possible range 230. In other words, there is a problem that in a relationship between a range (the region 250) in which data communication cannot be performed even though the index sensor 222 receives the laser beam of the index laser 229 of the portable receiving terminal 202 and a range (the transmission possible range 230) in which communication can be performed, the region 250 is relatively larger.
Also, in the conventional content delivery system 200, the terminal transmitter 203 has to include the index sensor 222 and the light 223. This makes a device larger in size and increases manufacturing cost of the device.